Procedure for the manufacture of oxalate coatings on metals



United State Fatent O PROCEDURE FOR THE MANUFACTURE OF OXALATE COATINGSUN METALS Werner Rausch and Franz Gonnert, Frankfurt am Main, Germany,assignors to Farker Rust Proof Company, Detroit, Mich, a corporation ofMichigan No Drawing. Application March 17, 1954 Serial No. 416,962

7 Claims. or. 148-614) The present invention relates generally to thechemical coating of metals such as iron, carbon steel, and stainlesssteel. More particularly, the invention relates to a complete methodincluding pretreatment of the metal and the application of such coatingsfrom solutions of oxalic acid. The method to be described herein isparticularly applicable to the coating of stainless or alloy steel-s tofacilitate drawing, spinning, and other metal forming operations.

In the manufacture of oxalate coated iron, steel and stainless or alloysteel articles, it is essential that a perfectly clean surface beexposed to the bath. When the metal is rusted and scaled, it has beentreated heretofore, prior to immersion in the oxalic acid solution, witha de-rusting or de-scaling acid pickling treatment, wherein the rust orscale layers are removed. It has also been reported that with thestainless or alloy steels, a further pretreatment in a bath of moltenalkali, following the acid pickling treatment, results in a furtherimproved oxalate coating thereon. The molten alkali pretreatment,however, has the disadvantage that it requires special equipment, and inaddition, is dangerous to workers in the plant.

The chemical coatings formed on metals protect the metal surfacesatisfactorily during most forming operations. Great difficulty isfrequently encountered, however, duringwire-drawing operations eitherbecause apparently tightly adherent coatings flake off or are scrapedoff on the leading edge of the die or the coating does not seem to havethe ability to properly lubricate the metal While in the die. Thus, thecoating must not only be of a correct weight and be tightly adherent tothe metal surface, but also it must have the property to cling to theetal in spite of deformation of the metal and the intense rubbing actionand form a thin lubricating film between metal and die. With thestainless steels, coatings of this desirable type are difficult toobtain. coating both drawing speeds and the reduction-per-draw must bereduced, and a rough surface is usually obtained.

it has now been found that the Weight, quality and adherence of theoxalate coatings can be significantly improved if the metal, which hasbeen subjected to a prior acid pickling treatment or other equivalentcleaning pretreatment, is given an additional pretreatment in an aqueousalkaline solution, before applying the oxalate coating. Oxalate coatingsapplied to the so-treated metal are especially improved on the stainlessor alloy steels, and also with ordinary iron and carbon steels. Theimprovement in all cases includes the development of a thicker coating,a more tightly adherent coating and the elimination of flaked and loosecoatings. In addition, the oxalate coatings produced by this method havea signiiicantly improved lubricating quality which is especiallyvaluable in Wire-drawing or tube-forming operations.

While in most cases the alkaline pretreatment step of the presentinvention will be applied to metal surfaces which have been subjected toa prior acid-pickling treat- Without such a- Patented May 20, lo e?ment, it is also applicable to metal surfaces which have been flamecleaned or the like where such a treatment makes pickling unnecessary.When the flame cleaned metal surface is treated in the alkalinesolution, an improved, heavier and more adherent oxalate coating isobtained. Thus, the alkaline pretreatment step is not necessarilyrestricted to metals which have been subjected to an initialacid-pickling step, although its principal application appears to be onacid-pickled metal surfaces.

The alkaline pretreatment is conducted according to the invention bycontacting a rustand scale-free metal surface with an aqueous solutionof alkali, for example, sodium hydroxide, potassium hydroxide, sodiumcarbonate, or of any other strongly alkaline-reacting salts of thealkali-metal hydroxides such as tertiary-sodium phosphate. in the usualcase, however, a sodium hydroxide solution is much preferred. Thealkaline solution is dilute, any concentration between 5 and 50%producing an improvement in the oxalate coating step. More preferredconcentrations when using the hydroxides are between 5 and with bestresults being obtained at 10 to 15%. An improvement in coating weight ofat least 30 to 40% and frequently of as much as or more is obtained withan acid-pickled, alkaline-rinsed metal as compared to that obtained frommetal that has had only the acid-pickling treatment. A particularlypreferred alkaline bath of the invention contains additional smallamounts of one or more activating anions selected from the classconsisting of cyanide and thiocyanate. Very small proportions of eitheror both of the latter, for example, from as little as 02% to as much as1%, effect a further increase in the Weight of coating deposited in agiven time from a given oxalate solution, of from 50 to 100%. Morepreferred concentrations for the activating anion are from 0.06 to 0.5%.In all cases the adherence and lubricating quality of the coating aremarkedly increased irrespective of the Weight thereof.

The oxalic acid solution applied to metal surfaces having thepretreatment of the present invention is not critical. If the solutionproduces an adherent coating on the metal, the weight, quality andadherenceof coating obtained on an alkaline-rinsed metal surface will beimproved. The coating will be more uniform, finer grained, more tightlyadherent and more useful in metal forming operations, particularly inwire drawing or the mandrel drawing of tubes. Thus, the usualiron-containing oxalic acid solutions such as the oxalic acid-ironoxalate solutions activated by ferric ion may be utilized. Iron-freeoxalic acid solutions activated by halide ions, particularlysilicofluoride ions may also be used. The iron-containing solutions maycontain chloride, bromide, thiocyanate or ferricyanide ions and alsooxygen-containing sulfur compounds such as thiosulfates, sulfites andthe like. Both the ironfree and iron-containing solutions may containoxidizing agents such as sodium chlorate or bromate, sodium nitrate ornitrite, hydrogen peroxide, organic nitro compounds such asmeta-nitrobenzene sodium sulfonate and the like. Much preferred are theferric ion activated oxalic acid solutions containing 0.4 to 15% totaliron, at least 0.002% ferric ion and from about 2 to 16% of chloride1011.

The method of this invention is applicable to the treatment of thesurface of any metal selected from the class consisting ofiron,chromium, nickel, and alloys of at least any two of these metals. Thus,the method is applicable to ordinary soft iron, to carbon steels, tochrome-, nickelor chrome-nickel steels, to essentially pure chromium,essentially pure nickel, and to nickel-chrome or chrome-nickel alloyslow in iron, or entirely free thereof. An example of the high nickel-lowchromium-low iron alloys is Inconel X.

Several of the more important general types of alloysincluded in thisbroad definition are the austenitic, martensitic, and ferritic stainlesssteels. The 'austenitic stainless steels contain a major proportion ofiron, from 16 to 26% chromium, and from 6 to 22% nickel, with the totalof the alloying elements being at least 23 The martenistic stainlesssteels contain from 4 to 18% chromium and nickel, if present, not morethan 2 or 3%. The ferritic stainless steels differ slightly from themartensitic since they usually contain slightly greater amounts'ofchromium, no nickel and a relatively greater variety and higherproportions of other alloying elements.

Other alloys which may be coated by the method of this invention containmajor proportions of nickel, minor proportions of chromium and little orno iron. The latter are unusually resistant to corrosion and are usedfor making clad steel sheets. A fuller description of the compositionand properties of these and other corrosionresistant metals included inthe above class may be found in the ASTM Metals Handbook 1948 or lateredition. All of these and others are satisfactorily coated by the methodof this invention. The stainless steels, since they are relativelydifficult to coat, are the most important class of alloys to which theinvention may be applied. With the especially diflicult-to-coataustenitic stainless steels, the greatest improvement in coating qualityand weight is obtained.

In the method of this invention, a metal surface should have at leasttwo pretreatment steps before the oxalate coating is applied. The firststep is a pre-clean step which may consist of any treatment adapted toremove soil, grease and rust or scale. This first pretreatment step mayconsist in a flame cleaning, solvent washing, molten salt treatment and/or an acid-pickling treatment. If the metal is not rusted or sealed, asimple flame treatment or solvent wiping or washing to remove all tracesof grease will be sufiicient. If rusted or sealed, the metal surfaceshould be pickled in acid. For the latter purpose, exceptionally thickand adherent scale layers can be loosened by a molten salt treatment,for example in Hookers salt and the so-treated surface then pickled inacid. The acidpickling solution will consist of dilute solutions ofstrong mineral acids such as nitric acid, hydrofluoric acid, sulphuricacid, hydrochloric acid and others, and mixtures of these and otheracids, or solutions of these acids with soluble salts such as sodiumchloride, ferric sulphate, and others. Preferred rust and scale removingpickling baths are as follows, in order of their preference:

(2) HNO HF 3 HNO HCI (4) H SO NaCl (with or without HNO The abovepickling solutions are applied to the metal at total concentrationsbetween 5 and 20%, more preferably about and at temperatures between 68and 176 F., preferably at ordinary temperatures of from 68 to 90 F. Theferric sulfate of bath (1) is used in concentrations of from 4 to 7%together with 1% to 2% or more hydrofluoric acid. The time of immersionwill vary considerably according to the bath strength and temperaturewith efficient cleaning being usually obtained in from 5 to 60 minutes,more preferably to 50 minutes.

The thus-cleaned metal preferably is first given a thorough Waterrinsing before being subjected to the alkaline pro-rinse treatment ofthis invention. Improved coatings are obtained when the metal is left incontact with the alkaline rinsing solution for as little as one minute.No significant improvement is obtained beyond about 15 minutes contact.The alkaline rinsing bath may be at any convenient room temperature andmay be warmed slightly, if desired. Temperaturesof from 84 to 194 F. aresatisfactory,'with 140 to 194 being preferred;

Following the alkaline prerinse treatment, the metal is again thoroughlyrinsed in cold water and then immersed in a conventional oxalate coatingbath. The latter bath is applied to the metal at any convenienttemperature from room temperature F.) to about 190 F. Practicalprocessing times of from 1 to 15 minutes are obtained at temperaturesbetween and F. After rinsing thoroughly with water, it is preferred toimmediately dry the oxalate coated surface in order to reduce the attackof the corrosive atmosphere usually prevailing in the pickling room. Theresulting coating is a suitable paint base and it protects the metalagainst corrosion and during metal working operations.

The thus-coated metal usually should be lubricated before beingsubjected to metal working operations. Ordinary sodium soaps as well asthe alkaline earth and aluminum stearates may be applied dry or fromsolutions or emulsions and the surface again dried. The metal also maybe limed when such treatment is permissible. The metal is then ready forthe drawing, spinning or other metal forming operation.

The invention will now be more fully described with reference to certainspecific examples. The latter are intended as being merely illustrativeand not as restricting the invention. Unless otherwise specified,concentrations in the following examples and elsewhere herein areexpressed as percent w./vol.

Example I A series of tube sections of ordinary 18/8 chromenickelstainless steel were subjected first to an acidpickling treatment untilsmooth in an aqueous bath containing 11% nitric acid and 1.5%hydrofluoric acid, the temperature of the pickling bath being 113 F. andthe time of immersion being from 30 to 50 minutes. The pickled tubesections were then thoroughly rinsed in running Water. A number of theseacid-pickled sections were immediately dried and immersed for 10 minutesat 122 F. in an oxalate coating bath having the following composition:

Grams/liter Oxalic acid 196 Iron (total) 70 NaCl In all cases, thecoatings on these tube sections were irregular and there were clearuncoated areas or spots on the surface. The coated tubes (Tests A) wereset aside for determination of the total weight of coating.

Others of the acid-pickled and water-rinsed stainless steel tubesections were immersed for 10 minutes in a bath containing 15% of sodiumhydroxide and which was maintained at 176 F. The tubes were again rinsedwith cold water and oxalate coated in the same manner (Tests B). Thecoatings in this case, however, were complete, uniform, tightly adherentand considerably heavier than those of (A) above.

A third group of the acid-pickled, water-rinsed stainless steel tubesegments or sections were immersed in a bath containing 15% sodiumhydroxide and 1% potassium cyanide. As before, the bath temperature was176 F. and the time of immersion was 10 minutes. After rinsing anddrying these tubes were then oxalate coated as before (Tests C). Theresulting coatings again were heavy, complete, uniform and tightlyadherent.

A' last group of the acid-pickled, water-rinsed tube sections wereimmersed for 10 minutes in a bath at 68 F., the bath containing 15%sodium hydroxide and 1% potassium thiocyanate. After rinsing in waterand drying these tubes (Tests D) were oxalate coated as before. Thecoatings obtained, like those of Tests B and C, were heavy, complete,uniform and tightly adherent.

From various quantitative tests, the weight of the adherent coatinglayer on all these tube sections was determined. Summarized, the resultsare as follows:

Weight of coating mg./dm. (ave.)

From the above, it is readily seen that coatings in Tests It through Dwere complete coatings while those of Tests A were irregular and failedto completely coat the metal. In addition, all of the B-D coatings werefrom 53 to 84% heavier than that of Tests A. When the coated stainlesssteel tubes of Tests A through D were drawn, however, it was found thatthe coatings'of Tests A were highly unsatisfactory in that they were notadequately adherent to the metal and they flaked or were scraped off themetal when the latter were drawn through the die. The coatings of TestsB through D, by contrast, were sufficiently adherent to the metal to bea great aid in the tube drawing operation. With the latter, higherdrawing speeds were utilized and greater total reduction in crosssection was permitted.

Example ll Ordinary 18/8 stainless steel is subjected to two differentpreliminary acid-pickling baths. The pickling baths and conditions'wereas follows:

( 1 Ferric sulfate percent 6 HF do 1.5 Temperature F 158 Time minutes 60(2) Nitric acid percent 11 HF do.. 1.5 Temperature F 113 Time minutes 30to 50 After immersing in the pickling baths, the metal in each case wasthoroughly rinsed with cold water and then immersed for 8 minutes inalkaline rinse baths containing 10% sodium hydroxide and in most casesvarying amounts of either sodium cyanide or potassium thiocyanate. Afterthe alkaline rinse, the metal was again thoroughly rinsed with water andthen oxalate coated in a coating bath similar to that of Example I. Theweight of coatings on each panel was determined to be as follows:

From the above it is seen that in every case the pre-rinse in plainsodium hydroxide solution produced an improvement in coating Weight andthat all pre-rinsing solutions tested which contained 0.06 to 1% of NaCNor KCNS produced still further improvements in coating weight. As theconcentration of NaCN or KCNS in the pre-rinse was increased, the weightof coating increased rapidly to a maximum between 0.06 and 0.5% and thenslowly decreased, although in all cases the weight of coating obtainedwith the alkaline pro-rinsing baths was greater than those obtainedwithout such a treatment.

Example III In a similar fashion pre-rinsing treatments varying from 15to 50% by volume of sodium hydroxide and activated with 1% potassiumcyanide or potassium thiocyanate 'were applied to a number of commonGerman corrosion- Several bundles of 6 mm. coiled wire of RNO 18 Germanchrome steel (18% chromium) and Anoxin 4 (18% chromium, 10% nickel and2% molybdenum) were de-scalecl by pickling for six hours in diluteHCl-HF followed by a complete pickling pretreatment in 14% hitric acidand finally given a thorough rinsing in cold Water. Several of thesebundles were given an oxalate coating at 113 to 122 F. in a bath similarto that of Example I; The coatings were gray-green and thin.

The remaining bundles of pickled wire were immersed for 10 minutes at176 F. in a bath containing 15% sodium hydroxide plus 1% potassium orsodium cyanide. After rinsing in water, the bundles were then given anoxalate coating as before. The coatings obtained were dusty, yellowishto gray-green in color, and they were uniform and heavy.

When these two groups of round wire were subjected to drawing to anoctagonal profile greatly differing results were obtained. With the wirenot subjected to the alkaline pro-rinse, the wire turned sharp on thefirst extrustion. By contrast, the wire that had been alkalinepre-rinsed before being oxalate coated could be drawn from 5.5 mm. to3.3 mm. in octagonal profile and from 5.5 to 2.25 mm. in round profilewith a total reduction in area of 83%.

In a similar fashion 18/8 stainless steel wire was pickled in HNO -HFand then in HNO given a pre-rinse bath treatment in 15% NaOH containing1.0% KCN, and coated with oxalate as before. The coatings were dusty andalmost a gray-yellow in color. When drawn on multiple dry extrusionmachines (12 extrusions) with a dry soap lubricant, reduction of from0.8 to 0.28 mm. (total reduction 83%) were obtained. Without thealkaline pre-rinse the oxalate coated wire could not be drawn in thismanner.

While there have been disclosed certain preferred ways of carrying outthe invention, it is desired not to be limited solely thereto, and itwill be appreciated that the invention is susceptible to modification,variation and change without departing from the proper scope or fairmeaning of the subjoined claims.

What is claimed is:

1. A method of producing oxalate coatings on stainless steels, whichcoatings greatly improve metal-drawing operations, which methodcomprises subjecting the surface of the metal to a series of treatmentsin the order: 1) an acid-pickling treatment to remove corrosion andscale, (2) an alkaline rinse in a solution containing from 5 to 25% ofan alkali-metal hydroxide and from 0.02 to 1% of an activating ionselected from the 7 class consisting of cyanide and thiocyanate, and (3)contacting the resulting acid-pickled, alkaline-rinsed metal surfacewith an oxalate coating solution until a uniform and tightly-adherentcoating is form-ed thereon.

2. A method as defined in claim 1 and further characterized in that theacid-pickling treatment is conducted in a bath of nitric andhydrofluoric acids.

3. A method as claimed in claim 1 and further characterized in that thealkaline rinsing solution contains from to 25% sodium hydroxide and 0.06to 0.5% of an alkali-metal cyanide.

4. A method as claimed in claim 1 and further characterized in that thealkaline rinsing solution contains from 5 to 25% sodium hydroxide and0.06 to 0.5% alkali-metal thiocyanate.

5. A method of producing oxalate coatings on stainless steels, whichcoatings greatly improve metal-drawing operations, which methodcomprises subjecting the surface of the metal to a series of treatmentsin the order: (1) an acid-pickling treatment in an aqueous solutioncontaining ferric sulfate and hydrofluoric acid, (2) an alkaline rinsein a solution containing from 5 to 25 of an alkali-metal hydroxide and0.02 to 1% of an activating ion selected from the class consisting ofcyanide and thiocyanate, and (3) contacting the resulting acid-pickled,alkaline-rinsed metal surface with an oxalic acid solution containingferric ion activator until a uniform, adherent coating is formedthereon.

6. A method as claimed in claim 5 and further characterized in that thealkaline-rinsing solution contains from 5 to 25% of sodium hydroxide.

7. A method of producing oxalate coatings on metals selected from theclass consisting of iron, chromium, nickel and alloys of at least anytwo of these metals, which method comprises subjecting the surface ofthe metal to a series of treatments in the order: (1) an acidpicklingtreatment to remove corrosion and scale, (2) an alkaline rinse in asolution containing from 5 to of an alkaline compound selected from theclass consisting of the alkali-metal hydroxides and their stronglyalkaline-acting salts and from 0.02% to 1% of an activating ion selectedfrom the class consisting of cyanide and thiocyanate, and (3) contactingthe resulting acidpickling, alkaline-rinsed metal surface with anoxalate coating solution until a uniform and tightly-adherent coating isformed thereon.

References Cited in the file of this patent UNITED STATES PATENTS2,116,954 Singer May 10,1938

2,338,045 Leonard Dec. 28, 1943 2,577,887 Gibson Dec. 11, 1951 FOREIGNPATENTS 661,385 Great Britain Nov. 21, 1951 OTHER REFERENCES Starr:Finishing Magnesium, Metal Finishing, October 1952, pages 62-64.

1. A METHOD OF PRODUCING OXALATE COATINGS ON STAINLESS STEELS, WHICHCOATINGS GREATLY IMPROVE METAL-DRAWING OPERATIONS, WHICH METHODCOMPRISES SUBJECTING THE SURFACE OF THE METAL TO A SERIES OF TREATMENTSIN THE ORDER: (1) AN ACID-PICKLING TREATMENT TO REMOVE CORROSION ANDSCALE, (2) AN ALKALINE RINSE IN A SOLUTION CONTAINING FROM 5 TO 25% OFAN ALKALI-METAL HYDROXIDE AND FROM 0.02 TO 1% OF AN ACTIVATING IONSELECTED FROM THE CLASS CONSISTING OF CYANIDE AND THIOCYANATE, AND (3)CONTACTING THE RESULTING ACID-PICKLED, ALKALINE-RINSED METAL SURFACEWITH AN OXALATE COATING SOLUTION UNTIL A UNIFORM AND TIGHTLY-ADHERENTCOATING IS FORMED THEREON.